An analog-to-digital (A/D) converter is a circuit on the borderline between the analog domain and the digital domain which acts as an intermediary in the exchange of information between the two domains. As the name indicates, an A/D-converter converts or transforms analog input signals to digital output signals. An A/D-converter could be used for converting analog information such as audio signals or measurements of physical variables into numbers consisting of two-level digits or bits; a form suitable for digital processing. A/D-converters are found in numerous applications of all modern technologies. They are widely used in different fields of electronics and communication.
Many systems that use A/D-converters, such as powerful digital processing systems, demand high performance A/D-converters. Such high performance A/D-converters should be able to operate at high speed and with high accuracy.
A particular type of A/D-converter that operates at high speed is the pipeline A/D-converter. A pipeline A/D-converter is a discrete-time A/D-converter which comprises a number of fast and usually relatively simple bit generating and signal processing stages that are connected in cascade. In a 1-bit per stage architecture, each stage generates a single output bit.
However, conventional pipeline A/D-converters generally suffer from limited accuracy. It is known that offset errors due to imperfections in the circuit realizations severely influence the accuracy of conventional pipeline A/D-converters. In particular, pipeline A/D-converters of switched-capacitor type suffer from offset errors due to e.g. DC-offsets, low-frequency noise and clock induced charge injections. In pipeline A/D-converters which generate output signals of regular binary code, the offset errors propagate from stage to stage and accumulate in a strictly increasing manner during an A/D-conversion, thus limiting the accuracy of the converter and increasing the distortion. Relatively large differential and integral non-linearities will be introduced, and in the worst case scenario output codes might be missing.